There's a device called a chiller but they're very expensive. If you don't mind spending the money then look up "aquarium chiller" on google and see what I'm talking about. I think there's also some DIY ones if you're able to do something like that.

Otherwise put a powerhead in there pointing slightly towards the top of the tank to turn the water over and oxygenate it (it's true that warmer water holds less oxygen, the circulation will also help to cool it) and you can also float bags of ice in the water.